Keya Tang,Jincheng Wang

College of Chemistry and Chemical Engineering,Shanghai University of Engineering Science,Shanghai 201620,China

Keywords:Montmorillonite Nanostructure Chlorinated butyl rubber Polymer processing Composites

ABSTRACT Montmorillonite (MMT) was modified by ultrasound and castor oil quaternary ammonium salt intercalation method to prepare a new type of organic montmorillonite(OMMT).The surface structure,particle morphology,interlayer distance,and thermal behavior of the samples obtained were characterized.The modified OMMT was then added to chlorinated butyl rubber (CIIR) by mechanical blending,and a composite material with excellent damping properties was obtained.The mechanical experiment results of CIIR nanocomposites showed that the addition of OMMT improved their tensile strength,hardness,and stress relaxation rate.Compared with pure CIIR,when the content of OMMT was 5 phr(part per hundred of rubber),the tensile strength of the nanocomposite was increased by 677% and the elongation at break was also increased by 105.4%.The enhancement of this performance was mainly due to the dispersion of the nanosheets in CIIR rubber and the chemical interaction between the organoclay and the polymer matrix,which was confirmed by morphology and spectral analysis.OMMT also endowed a positive effect on the damping properties of CIIR nanocomposites.After adding 5 phr of OMMT,the nanocomposite owned the best damping performance,and the damping factor,tanδmax,was 37.9%higher than that of pure CIIR.Therefore,the good damping and mechanical properties of these CIIR nanocomposites provided some novel and promising methods for preparing high-damping rubber in a wide temperature range.

1.Introduction

To improve the service life and reliability of large and heavy equipment,vibration and noise reduction has become the focus of the dynamic design [1].The traditional damping pad has a low loss coefficient,low vibration damping effect,weak fatigue resistance,poor aging resistance,and it is difficult to replace later [2].Therefore,in the research of damping structure,the damping performance of materials in energy conversion and dissipation is fully utilized to achieve the effect of vibration and noise reduction[3,4].With the rapid development of the equipment manufacturing industry,the complexity of the equipment and the functions it realizes are increasing.This puts stricter requirements on design methods and manufacturing processes.Vibration control has become an essential part of equipment manufacturing that cannot be ignored.Among many methods of vibration and noise reduction,viscoelastic damping material was used to suppress vibration levels and reduce structural noise.This is the most cost-effective and fastest solution at this stage [5].

As an efficient and economical damping material,rubber has the characteristics of a polymer and is a composite material composed of various additives [6].Chlorinated butyl rubber (CIIR) is a derivative of isobutylene and isoprene (usually 1%-5%) reacted with halogen.They are favored by many researchers because of their good air permeability and moisture permeability,as well as their excellent ozone resistance and high chemical stability.Due to its fast vulcanization speed and better blending properties,it is often used as the first choice for studying damping materials[7,8].Chlorinated butyl rubber(CIIR)is a type of elastomer known to be a promising material for sound insulation and vibration reduction applications.Moreover,CIIR has a fast-curing rate,high cross-linking flexibility together with a sensitivity of thermal processing.The internal friction peak extends from 10 to -60 °C,becoming a damping material with a relatively wide active functional area.This has also expanded its application range and has a very important position in the industry,such as tire inner tube,medical rubber seal,chemical equipment lining,protective equipment,heat-resistant conveyor belts,etc.[7,9].Moreover,CIIR unique worm-like structure makes its own a dense molecular chain,and the side methyl groups on its molecular chain are very dense.It can be known that the more side groups on the molecular chain,the larger the volume,the stronger the polarity,and the more intermolecular hydrogen bonds,the better the damping performance of rubber[10].This is the main reason that CIIR possesses a high damping factor,high friction in the matrix together with high damping performance.However,the damping behavior of CIIR is not too good to meet the requirements of the industry,which is not conducive to its further application in specific fields[7].Liaoet al.[11]prepared a series of CIIR rubber composite materials.The loss peaks tended to shift toward high temperature,but the maximum loss factor (tanδmax) was not over 0.35.Liuet al.[6] prepared new composite materials by blending CIIR and terpene resin.Although the damping performance and mechanical properties were improved,tanδmaxonly increased by 0.2.Therefore,it was imperative to prepare CIIR composite materials with high damping factors and excellent mechanical properties.

In recent years,the design of nano-particle composite materials with excellent properties has received extensive attention [12].Elastomer nanocomposites have aroused great interest in the past two decades because of their excellent performances [13].By introducing a new polymer/filler interface,the molecular dynamics near the solid surface or even the entire polymer matrix can be changed,and thus proper fillers can further improve the performance of the elastomer [14,15].Nanofillers used in traditional elastomer nanocomposites are mainly concentrated in metal oxides such as nano calcium carbonate (CaCO3),nano titanium dioxide (TiO2),and nano zinc oxide (ZnO),as well as synthetic whiskers,carbon nanotubes and other rod-shaped fibers.Now researchers are beginning to design new materials with predictive properties through different functionalized nanoparticles[16].The choice of nanoparticles in different applications depends on their own properties[17].Hydroxyapatite(HA)nanoparticles can better improve the oil resistance and flame retardancy of the chlorinated ethylene-propylene-diene rubber[17,18].Metal nanoparticles zinc sulfide can greatly shorten the vulcanization time of rubber nanocomposites and improve the degree of dispersion in the matrix [19].Moreover,in order to better improve the divergence of nanomaterials,a large number of researchers choose to use the solution mixing method and melting method [18].But it has a bad influence on the environment and the mechanical properties of composite materials [20].

Layered silicate has been extensively studied by academia and industry due to its cheapness,easily available and environmentally friendly advantages.In particular,the typical representative of silicate is montmorillonite (MMT) [21,22].Many researchers have reported the use of organoclay to improve the mechanical,thermal,flame-retardant and damping properties of polymers [23].Alshabanatet al.[24] prepared polymer nanocomposites of polystyrene matrix containing 10%(mass)OMMT using the solution method.It was found that exfoliates were obtained when the interlayer spacing was sufficiently enlarged to overcome the interactions forces between the adjacent layers,which may be randomly dispersed in a continuous polymeric matrix.This unique structure makes it have a very high aspect ratio and a large specific surface area.When added to the polymer matrix,the internal friction increases with the increase of the interface area[25].Chanraet al.[26]used miniemulsion polymerization method to fabricate the hybrid OMMT/PMMBA latex with different amount of OMMT.The barrier property and thermal stability performance of the hybrid PMMBA latex was improved by the addition of OMMT.This was attributed to effectively dispersing and exfoliating of MMT in the polymer matrix.Chenet al.[25]enhanced the physical properties especially the damping capacity of polyurethane (PU)/epoxy resin (EP)grafted interpenetrating polymer network (IPN) composites by introducing MMT,and found that the internal friction increased with the increasing interface area,and this can increase the energy dissipation in the system and greatly improved the damping performance.Wanget al.[27] explored the theory of nano-layer reinforcement for elastomer.In EPDM/MMT nanocomposites,a large number of polymeric chains were distributed between the layered silicate layers due to the particularity of EPDM structure.This caused a strong interaction between the rubber and the MMT nanolayers.As a result,the internal energy consumption of the matrix was increased,and the matrix exhibited better mechanical and damping properties.Therefore,the polymer/MMT composite material model is widely used in the research of damping materials [23].

It can be clearly seen from above literatures that in the research of improving mechanical properties of elastomer nanocomposites,there are few studies on the damping performance.In addition,the realization of nano-level dispersion of nano-layered silicates in the matrix is also rarely reported.In this study,a two-step method was used to modify MMT.It was proposed for the first time to use a cell pulverizer to process natural MMT,and MMT sheets with a singlelayer structure were successfully prepared.Meanwhile,the double quaternary ammonium salt method was used to organically modify MMT to prepare CTAB/RABB-MMT.Then,a series of CTAB/RABB-MMT were incorporated into CIIR rubber matrices,and CIIR/OMMT nanocomposite materials were obtained.Meanwhile,the relationship between the tensile strength,elongation at break and other mechanical properties and the clay content of the composite materials was studied.In addition,the effects of different doses of OMMT on the dynamic mechanical properties,loss modulus and storage modulus of composite materials were investigated.

2.Materials and Methods

2.1.Materials

Na+-montmorillonite (Na+-MMT,industrial grade,CEC=90 mmol/100 g (cation exchange capacity),layer spacingd=1.49 nm (2θ=5.9°)) was purchased from Fenghong New Chemical Materials Co.,Ltd.(Zhejiang,China).Chlorinated butyl rubber (CIIR,Mn=255800 (number average molecular weight),PDI=1.72(polydispersity index),Exxon 1066)was purchased from Exxon Mobil Chemical Industry Company (Shanghai,China).Ricinoleic acid (RA) was obtained from the exploration reagent platform (Shanghai,China).It has a density of about 0.94 g·cm-3,a boiling point of 416.4 °C,a molecular weight of 298.5 and a purity of 80%.1-(3-dimethylaminopropyl)-3-ethyl-carbodiimide hydrochloride (EDCI),4-dimethylaminopyridine (DMAP),3-dimethylaminopropylamine and dichloromethane (DCM),bromoethane and cetyltrimethylammonium bromide (CTAB) were supplied by Aladdin Reagent Co.,Ltd.(Shanghai,China).All the rubber additives used in this study were obtained from Tansoole Company (Shanghai,China).

2.2.Preparation of organoclay

2.2.1.Synthesis of quaternary ammonium salt

RA(2.9 mmol)was dissolved in anhydrous DCM(0.2 mol·L-1)at room temperature and in a nitrogen atmosphere.N,N-alkyldiamine(4.3 mmol),EDCI(4.3 mmol),and DMAP(0.9 mmol)were added successively and stirred for 2 h in an ice water bath.Then,the mixture was stirred at room temperature for 8 h.After the reaction was completed,NH4Cl solution was poured into and extracted with DCM (3 L).The organic layer was washed using brine,dried over anhydrous Na2SO4,and concentrated in a vacuo condition.In a nitrogen atmosphere,the amide (0.8 mmol) was dissolved in anhydrous acetonitrile (0.3 mol·L-1),and alkyl halide(1.6 mmol) was slowly added drop-wise to the solution.The mixture was heated to 140°C and stirred for 7 h.After the reaction,the mixture was concentrated in vacuo after cooling to room temperature.

2.2.2.Synthesis of quaternary ammonium salt

Na+-MMT was modified in two steps.As shown in Fig.1,the first step is to modify the MMT by physical ultrasonic method.Na+-MMT was added to a 500 ml beaker,and deionized water was poured in to prepare a 3%mass solution.The mixture was stirred vigorously for 1 h to form a flocculated precipitate.Then,it was put in an ultrasonic cell pulverizer,and was ultrasonic pulverizating at 95%power for 24 h.When it turned into a milky white solution,it was placed in a 500 ml flask and stirred vigorously at 60°C for 24 h.Then,the ultrasonic pulverization was continued for one day at a power of 95% at 60 °C to make it turn into an off-white solution.The modification by the physical ultrasonic method was completed,and the obtained off-white solution was labeled as solution A.

The second step is to use alkane quaternary ammonium salt to intercalate modified MMT.First,RABB was dissolved in deionized water to prepare an aqueous solution with a mass fraction of 10%,and then it was heated to 60 °C under vigorous stirring and was recorded as solution B.CTAB was added with deionized water into a beaker to obtain solution C.The solution A,C and B was added to a flask in turn and stirred magnetically at 60 °C for 8 h.After the reaction was over,the mixture was poured into a beaker and washed with 50% aqueous ethanol until the supernatant was free of Br-(tested with 0.1 mol·L-1AgNO3).Then,the OMMT slurry was poured into a beaker with 2 L deionized water.After vigorously stirring,the upper layer of organic smectite foam was taken out.It was placed in a freeze dryer for two days,and finally OMMT was obtained.

The two-step modification of MMT was shown in Fig.2.The layered structure of pure MMT was obvious,evenly dispersed in the solution.After ultrasonic cell smashing,the layer spacing was enlarged by local high temperature and high pressure,and the single-layer MMT sheet was peeled off.As shown in the figure,the peeled off single-layer MMT was dispersed in the gap of the multilayered MMT.At this time,due to the opening of the layer spacing,the free metal cations between the layered silicates were further exposed,and the organic ammonium ions with long chains may be more easily exchanged with the metal cations.

2.3.Preparation of organoclay reinforced composites

The gum of CIIR was put into a double roller rubber refiner(XK-1600,Shanghai Second Rubber Machinery Factory,Shanghai,China),and then it was plasticized for 20 min.According to the formula provided in Table 1,a certain amount of stearic acid and magnesium oxide was added to the CIIR matrix.After 4 min of plasticization,the nano clay(0,1,3,5,7,and 10 phr(part per hundred of rubber)) was added to the mixture.Then,the mixture was plasticized in the open mill for 20 min.After taking out and cooling completely,accelerator,zinc oxide and sulfur were added in sequence.After fully mixing to prevent pre-vulcanization,the composite was immediately taken out and cooled for 8 h.The curing time of this composite was measured at 160 °C with a rubber vulcanization analyzer.After that,it was heated and solidified in a flat vulcanizer,and the OMMT/CIIR composite was obtained after cooling.

Table 1Formulation of different CIIR composites prepared by MMT and OMMT

2.4.Characterization

2.4.1.FTIR analysis

The infrared spectra of MMT and OMMT were measured by an infrared spectrometer (FT-IR370,Nicolet Co.,USA).The samples were dried before testing.The scanning range was from 4000 to 400 cm-1,and each sample was scanned 16 times to determine the groups and structures of the different compounds.

2.4.2.1H NMR analysis

The quaternary ammonium salt and intermediate products were tested by1H NMR using a nuclear magnetic resonance instrument.The test solvent was deuterated chloroform,the scanning frequency was 400 MHz,and the testing instrument was Bruker AV600 spectrometer (Bruker,USA).

2.4.3.XRD analysis

The interlayer spacing of MMT before and after modification was analyzed by wide-angle X-ray diffraction (XRD) method.It was performed using a Rigaku D-Max/400 X-ray diffractometer(Ricoh,Japan).X-ray scanning was from 1° to 10° (2θ) at a rate of 2 (°)·s-1.The interlayer spacingdwas calculated using the Bragg equation:d=nλ/2sinθ,where 2θ was the angle between the incident X-ray and the corresponding crystal plane,λ was the wavelength of the X-ray,andnwas the integer wavelength number(n=1).

2.4.4.SEM observation

The surface and interior morphology of MMT clay powders and CIIR rubber composites were tested using a Hitachi S-2150 scanning electron microscope (SEM,Hitachi,Japan).An electron beam potential of 25 kV was used,and the surface of the samples was sprayed with gold.The energy spectrum analysis (EDS) of MMT,OMMT and CIIR/OMMT composites was conducted using the genesis type energy spectrum analyzer (EDAX,USA).The energy spectrum of the different elements can be used to characterize and analyze various components.

Fig.1.Synthesis of aliphatic amido-quaternary ammonium salts:(a) castor oil amide,(b) quaternary ammonium salt of castor oil.

Fig.2.Microscopic schematic diagram of two-step modified MMT.

2.4.5.POM observation

The cross-section morphology of the CIIR nanocomposite was observed using a polarizing microscope (POM,XPF-300,Shanghai Tianxing Instrument Co.,Ltd.,China).The magnification was 0 to 400 times and the test was conducted at room temperature.

2.4.6.TEM observation

Morphology observations for intercalation and exfoliation of layered OMMT and CIIR nanocomposites were operated by a transmission electron microscopy (TEM) (JEM-2100F,JEOL,Japan) with an acceleration voltage of 200 kV.All samples with the thicknesses of about 100 nm were cut assisted by a Leica EMUC6/FC6 microtome.

2.4.7.TG analysis

The thermal properties of MMT,OMMT and CIIR rubber composites were tested using a STA PT100(Linseis,GER)thermogravimetric analyzer in a nitrogen atmosphere from room temperature to 800 °C with a heating rate of 10 °C·min-1.The quality of the sample was controlled at 10 mg.

2.4.8.Mechanical properties

The tensile properties of CIIR rubber were measured according to GB 528-82.The rubber was cut into a dumbbell sample with a size of (115 mm ± 0.1 mm) × (6.0 mm ± 0.1 mm) × (2 mm ± 0.1 mm).The universal tensile tester (TCS-2000,Dongguan,China)was used at room temperature,and the stretching rate was 500 mm·min-1.Hardeness was tested using a Shore Durometer(XY-1,Rubber Hardness Tester,Rubber Chemical Machinery No.4,Shanghai,China).The compressive stress relaxation characteristics of the CIIR composites were measured according to GB 1685 using a compressive stress relaxation instrument (KRT-1007,Shanghai,China) with a 9 mm thick cylindrical pattern.The compression set coefficient was calculated byk=(F0-Ft)/F0(wherekwas the compression set coefficient;Ftwas the compressive force at constant temperature agingttime;F0was the Initial compression force).

2.4.9.Dynamic mechanical analysis

Dynamic mechanical analysis was carried out using dynamic thermomechanical analysis (DMA,Q800,USA).The size of the specimens was 12 mm × 6.5 mm × 2 mm.The test sample was measured for storage modulus (E’),loss modulus(E′′) and loss factor(tanδ)from-120 to 200°C with a temperature increase rate of 3 °C·min-1under liquid nitrogen conditions.The strain amplitude was 0.15%,and the frequency was 1 Hz.

3.Results and Discussion

3.1.Analysis of modified montmorillonite (OMMT)

The preparation process of OMMT was depicted and analyzed as shown in Fig.3.The two-step modification method of MMT was divided into ultrasonic physical modification and surface cation exchange methods.A microscopic diagram of MMT modification was shown in the left side,and a schematic diagram of MMT macroscopic modification was exhibited in the right side.As shown in the right side,when MMT was dissolved in water,due to hydration,water molecules entered into the middle of the mineral crystal lattice.The macroscopic appearance was that MMT can be suspended and dispersed in water.The dispersed MMT slurry was put into an ultrasonic cell pulverizer.Due to the weak interaction between the MMT layers,they were exfoliated and dispersed under the action of local high temperature and high pressure.Meanwhile,organic cations may be inserted between the interlayer spacing of MMT,and thus the hydrophilic MMT was converted into lipophilic MMT using the cation exchange method.This was an important basis for nano-dispersion of organoclay in the CIIR matrix.CTAB and RABB were used as organic modifiers.CTAB performed ion-exchange first to open a part of the interlayer spacing of MMT,and then RABB modified it further to open up the interlayer spacing on a deeper level.It can be seen from the micrograph that the surface of the dispersed layered silicates was covered by long chain alkyl carbon chains,which may increase the affinity of organoclay with the organic phase.In addition,the longer alkyl molecular chains of CTAB were intercalated between the layers,increasing the interlayer spacing,facilitating the insertion of RABB,and thus forming a CTAB/RABB-MMT intercalation structure.

3.2.Structure,morphology and properties of OMMT

Fig.3.Two-step modification process of organic MMT.

Fig.4.1H NMR spectra of (a) RA,(b) RABB.

1H NMR analysis.Fig.4 presented1H NMR of RA and synthesized RABB according to Fig.1.It can be seen that the spectral analysis of amides and quaternary ammonium compounds was consistent with the expected structure[28].From RA,amide compounds to quaternary ammonium salts,the1H NMR spectrum of each series of compounds was similar.In the amidation reaction,as shown in Fig.4(a),the hydroxy signal of RA(δ=11.0)disappeared and was replaced by the chemical shift at δ=8.23.The proton on the amide indicated the presence of -CONH in the target compound.The hydrogen at δ=2.29 was attributed to the group of methyl carbons in propyl dimethylamine,and the hydrogen on carbon of propyl dimethylamine connected to the amide appeared at δ=3.33.Meanwhile,there was a chemical shift of the proton on the double bond at peaks around δ=5.40.The chemical shifts of the remaining methylene groups and methyl carbon-hydrogen on the castor oil-based framework were distributed between δ=0.89 and 2.46[29].As shown in Fig.4(b),in the quaternization reaction,the chemical shifts at δ=3.27 and 3.30 were attributed to the hydrogen of -CH3and -CH2on the quaternary ammonium salt [30].The chemical shift at δ=8.47 was the hydrogen on the amide nitrogen atom,and the others were chemical shifts of hydrogen on RA [31].Above analysis demonstrated that the reaction shown in the chemical formula (Fig.1) was successfully proceeded.

Fig.5.FTIR spectra of MMT and OMMT.

FTIR analysis.In order to understand the modification of the layered silicate,Fourier transform infrared spectroscopy was applied to analyze and understand the chemical interaction between the layered silicate and the compound [24].In Fig.5,two characteristic peaks appeared between 3500 and 3700 cm-1in the structure of MMT [26].One was relatively sharp and the other was relatively passivated,and this indicated that two types of hydroxyl groups existed in the inorganic silicate,namely solidphase and free hydroxyl groups.However,the characteristic absorption peak at 3500 cm-1was not found in the other three infrared spectra,which indicated that the free -OH groups disappeared in the modified and freeze-dried OMMT [32].In RABB,the increase in the peak depth at 1004 and 684 cm-1was related to the stretching and bending vibration of Si-O-Si in layered silicates and the bending vibration of=C-H,respectively.Two obvious absorption peaks appeared at 2700-2900 cm-1,which were the stretching vibration peaks of -CH3and -CH2groups.This illustrated that the organic group had completely entered the interlayer of MMT[33].It can be seen that the corresponding peak in the spectrum of CTAB/RABB-MMT was higher,which indicated that the organic groups in the interlayer intercalation agents was more and they were all from -CH3and -CH2in CTAB and RABB.In addition,the peak at 1478 cm-1corresponded to amide group() of the amine chains which entered into the clay galleries.The exhibition of secondary covalent network was obvious from the increase in peak intensity at 1678 cm-1corresponding to stretching vibration of C=C aromatic bond in RABB.The peak at 1728 cm-1was bending vibration peak of -OH groups,which was related to the hydroxyl vibration peak of RABB in the long chains.In summary,it was shown that ion exchange occurred between the interlayers,and the quaternary ammonium salt long chain macromolecules were inserted between the MMT sheets.The above analysis may prove that CTAB/RABB-MMT was successfully prepared.

XRD analysis.The influence of crystal structure of the organoclay on damping property is very important.Here,XRD is used to analyze the microstructure,the interlayer spacing and interlayer intercalation of clay.MMT and OMMT can show characteristic peaks in XRD analysis due to their layered structure.High and sharp characteristic peaks generally appeared in the diffraction pattern of MMT with large layer spacing and a good degree of interlayer peeling.According to Linet al.[34],organic modification can change the interlayer release or interlayer spacing of the clay lattice,which may cause changes in the position,width,and intensity of the characteristic peaks in the XRD curves.In Fig.6,the characteristic peak of the original MMT appeared at 2θ=5.92°corresponding to the interlayer space of 1.49 nm.The XRD diffraction spectra of different organically modified intercalation showed that the diffraction peaks were lower than that of pure Na+-MMT.The spacing of CTAB alone intercalated MMT layers can reach 4.18 nm,and the spacing of RABB single intercalated ones can reach 3.19 nm.It showed that the interlayer structure of MMT was effectively changed and the inter-layer spacing was increased due to the effect of intercalation or exfoliation.As shown in Fig.6,2θ of CTAB/RABB-MMT was 1.98°,which corresponded to the interlayer space of 4.46 nm.This illustrated that the interlayer spacing of OMMT was fully enlarged.Through the above analysis,it was preliminarily proved that the preparation of OMMT was successful from the microscopic perspective of the inter-layer spacing.Moreover,the large interlayer spacing of OMMT also played a decisive role in its ability to be added as a nanofiller to the CIIR matrix.

Fig.6.XRD patterns of pure MMT and OMMT.

SEM analysis.SEM can more intuitively observe the morphology and under-stand the modification status of organoclay.Fig.7 presented the SEM images of MMT before and after the modification.It can be seen from Fig.7(a) that the unmodified MMT obviously possessed larger particles,which were easy to agglomerate and form a disordered structure[35].However,after the ultrasonic modification,the intercalation modification of the MMT using organic quaternary ammonium salt was performed.From Fig.7(b),the sharp angles on the surface became obvious,and the flaky structure was also more obvious.These were due to the slow enlarging of the CTAB modified MMT layer.From Fig.7(c),the sharp corners of the MMT were more visible,and the sliding between the layers was greater.Due to the increase of the interlayer spacing,the morphology of CTAB/RABB-MMT showed a semitransparent sheet structure [33].After the introduction of CTAB and RABB,a large amount of ion exchange occurred inside the MMT,the strong polarity became weak,the interlayer force was reduced,and the interlayer distance became larger,which caused a huge change in the inter-layer environment of the modified MMT.This was more conducive to the dispersion of OMMT in the rubber substrate,and it was easy to prepare composite materials with excellent performance.This can provide the possibility of expanding the application field of OMMT and enhancing the value of MMT [25].

Fig.7.SEM images of (a) Na+-MMT,(b) CTAB-MMT and (c) CTAB/RABB-MMT.

Fig.8.TG (a) and DTG (b) curves of MMT and OMMT.

TG analysis.A technique based on thermal analysis can be used to analyze the interface interaction between the layered silicates and the polymer chains.The thermal analysis data of OMMT determined by TG and DTG data were presented in Fig.8.It showed that when the modified clay material was heated,the mass loss was from 70%to 80%,and three decomposition stages existed.The first stage occurred from 30 to 120°C,and was attributed to the loss of water absorbed by MMT ranging from 3%to 6%.The decomposition temperature of the second stage was between 165 and 350°C,and the corresponding mass loss was between 30% and 40%.The thermal mass loss at this stage was very obvious as can be seen from Fig.8.The thermal behavior of CTAB-MMT can be seen in the DTG curves.The mass loss rate was the fastest at 259.2°C.This was caused by the thermal decomposition of the polymer on the surface and edge of the organoclay.In addition,the RABB-MMT had the highest thermal decomposition rate at 308 °C.This was also due to the decomposition of the polymer inside the interlayer sandwich after the outer polymer was decomposed.In the third stage,the decomposition temperature was from 400 to 650 °C,and the mass loss range was between 20% and 30%.On one hand,this was due to the further decomposition of the intercalation polymer and certain salts remaining between the MMT layers.On the other hand,it may be the collapse of the crystal structure caused by the loss of OH groups in clay minerals.Thermal analysis technology accurately reflected the interlayer situation of the organism entering the MMT.At the same time,it also helped to confirm that the MMT layer did slip and exfoliate due to polymer intercalation and ultrasonic pulverization.It also provided the possibility for MMT to play a good damping effect after entering the CIIR matrix.

3.3.Mechanical properties of CIIR/OMMT composites

Fig.9 showed the mechanical properties of CIIR composites with different ratios of MMT and CTAB/RABB-MMT.The addition of nanoclay endowed different degree of influence on the elongation at break (EB) and tensile strength (TS) of CIIR composites.It can be seen that with the addition of nanoclay,the tensile strength of CIIR composites increased regularly.On the one hand,it was caused by the strength of the clay itself,which exhibited a very good reinforcement effect on CIIR materials.On the other hand,it was caused by the slippage of the rubber molecular chains and the orientation of the layered silicates under tensile force.This type of slippage made it possible for the highly strained molecular chains to relieve the tension caused by stretching,and thus it may lead to higher breaking strain and strength [36].Moreover,it can be seen in Fig.9(a)that the tensile strength of the composite material with CIIR/CTAB/RABB-MMT was much higher than that of the composite material with only MMT.This was due to the huge specific surface area of CTAB/RABB-MMT,and thus the interface between the sheet layer and the rubber matrix became stronger.In addition,the degree of orientation and crystallinity of the macromolecular chains was increased under stretching,which led to the substantial improvement of the composite materials.This was very consistent with the properties of clay nanocomposites studied by Wuet al[37].

As shown in Fig.9(a) and (b),when pure MMT was added to CIIR,the TS value first increased and then decreased as the clay content increases,while the EB value was slightly increased.As for the CIIR composites with OMMT,after adding 5 phr of OMMT to CIIR matrix,TS and EB showed the same decreasing trend.This may be due to the dispersion of clay in the CIIR matrix.There were many reasons that affect dispersion.First,it was caused by the polarity difference between the organically modified clay and the origin clay.Second,it was resulted from the influence between the organically modified clay and the CIIR polar segment.The better the dispersion effect,the better the performance of the composite material.This was also the reason why EB decreased after adding 7 phr OMMT to CIIR.This was because,with the addition of CTAB/RABB-MMT,a large number of layered structures were generated inside the vulcanizate,which made it store more energy in the process of elastic deformation and consume a lot of energy during the deformation process [38,39].

Fig.9.Mechanical properties of different CIIR composites:(a) tensile strength,(b) elongation at break,(c) hardness,(d) compression set.

The hardness gradually increased after the addition of CTAB/RABB-MMT(Fig.9(c)).Compared with CIIR/MMT composite material,CIIR/CTAB/RABB-MMT hardness was obviously larger.This was because MMT was a natural mineral material,and its hardness was higher compared to CIIR.Thus,when added to form a composite material,it had a tendency to increase the hardness of the material.When 1-5 phr of CTAB/RABB-MMT was added to CIIR,the quaternary ammonium salt on the sheet promoted cross-linking,resulting in an increase in the cross-linking density of the composite material.In addition,the sheet layer acted as a cross-linking point in the rubber matrix,and the cross-linking density of the composite material was increased.These led to the gradual increase in the hardness of the composite material.However,compared with the pure MMT in the same proportion,the hardness of CTAB/RABB-MMT reinforced rubber was decreased when 7 and 10 phr was added.This may be due to the distinct plasticizing effect of CTAB/RABB-MMT,which may increase the chain flexibility of the rubber matrix [25].The compression set of the composite materials with CTAB/RABB-MMT was much higher than MMT (Fig.9(d)).On the one hand,rubber underwent self-polymerization during high-temperature fatigue aging;on the other hand,the graft polymerization or hydrogen bonding between MMT and the molecular rubber chain caused the rubber phase to be bound.The weakening of resilience led to an increase in permanent compression deformation and a weaker degree of reversible recovery[40,41].Therefore,the movement ability of the chain segments was weakened,and thus the recovery of the sample during compression deformation was small and the compression set was large.

3.4.Damping properties of CIIR/OMMT composites

Fig.10 presented the loss tangent (tanδ),storage modulus (E’)and loss modulus(E′′)of CIIR and different OMMT/CIIR composites.The damping characteristics of the materials were based on the various groups within the materials[16].Compared with pure CIIR,after adding CTAB/RABB-MMT,the glass transition temperature,Tg,of modified CIIR composite (corresponding to the tanδ peak)moved to the high-temperature region and the peak value became higher (Fig.10(a)).When the mass content of OMMT was 5 phr(the mass ratio of CIIR to OMMT is 100:5),tanδ reached the maximum value of 1.47,and the corresponding temperature was around -7 °C.After adding 5 phr of OMMT,the damping factor was 37.9% higher than that of pure CIIR,1.06.This may be related to the special layered structure of OMMT.The nano-level dispersion of OMMT in the polymer matrix led to high interface contact between the polymer and the inorganic filler.This can make its performance better than the polymer bulk phase [16].Moreover,the hydrogen bonds in the OMMT and CIIR composites endowed an impact on the dynamic mechanical performance,thereby improving the damping performance of the composite materials.It was worth noting that due to the addition of OMMT,the position of the tanδ peak shifted to high temperature,that is,from-22°C of pure CIIR to -7 °C of the composite material.It may be that when OMMT was added to CIIR,the friction between the OMMT and the polymer chains and the friction between the MMT itself increased the degree of energy dissipation[25].The damping performance of a material was actually related to the rate of energy dissipation.Internal friction was one of the measures of energy dissipation.The stronger the internal friction,the higher was the damping capacity of the material.Chenet al.[42]improved internal friction by preparing polyurethane(PU)/epoxy resin(EP)grafted interpenetrating polymer network (IPN) composites.Agrawalet al.[43]used graphite flakes and nano tungsten disulphide to change the friction properties of polymers/fillers.Although the damping performance has been improved,the improvement is not significant.These nano-scale materials had their own limitations,resulting in a limitation in the degree of dispersion.However,the layer structure of OMMT itself can approach two-dimensional materials without limitation.This was also the reason why CIIR/OMMT possessed high damping performance.In Fig.10(b),it can be seen that theE” of the composite materials tended to increase with the increase of CTAB/RABB-MMT content.In Fig.10(c),the peaks of allE’curves increased with the increase of CTAB/RABB-MMT’s content.The larger the nanoparticle loading,the higher the storage modulus,which was not only due to the formation of more filler networks but also due to the decrease in the volume content of the matrix polymer.As the temperature increased,the decrease of the storage modulus indicated that the flexibility of the composite material increased.Moreover,there was only one transition,which illustrated that there was no phase separation in the complex polymer network,and in which a strong hydrogen bond interaction may exist.

Fig.10.Dynamic mechanical properties of CIIR composites:(a) tanδ,(b) loss modulus,(c) storage modulus.

3.5.Microstructure of composite materials

The primary condition for enhancing and improving the mechanical and damping properties of nanocomposites was the dispersion status of nanofillers in the matrix.Therefore,the fracture morphology of pure CIIR and its composite materials was observed by SEM.For pure CIIR (Fig.11(a)),the fracture was smooth,and there were small particles on the surface of the substrate.These particles may originate from rubber accelerators and metal oxides (ZnO and MgO) used in rubber vulcanization,as well as fractures and minimal deformation.Fig.11(b) was the image of a CIIR composite with 5 phr of MMT.It can be found that the polymer structure was destroyed.Due to the aggregation and dispersion of MMT,the port appeared uneven and not smooth.However,after adding 5 phr of OMMT,it can be observed that the compatibility of OMMT with the CIIR matrix was better(Fig.11(c)).Their regular and rough surface in the fractured CIIR phase may be ascribed toa better physical inter-action between filler and polymer in the substrates.From Fig.11(c),it can be seen that there was obvious flaky clay released in the CIIR matrix,and a higher resistance to fracture propagation was formed.The results showed that the dispersion effect of OMMT reinforced CIIR composite material was more uniform than that of the untreated MMT reinforced CIIR matrix.

Fig.11.SEM of (a) CIIR,(b) CIIR/MMT (5 phr),(c) CIIR/CTAB/RABB-MMT (5 phr).TEM of (e) CIIR/CTAB/RABB-MMT (5 phr),(f) CIIR/CTAB/RABB-MMT (7 phr).

Fig.12.POM of fracture sections of different CIIR composites:(a)pure CIIR,(b)CIIR/CTAB/RABB-MMT(1 phr),(c)CIIR/CTAB/RABB-MMT(3 phr),(d)CIIR/CTAB/RABB-MMT(5 phr),(e) CIIR/CTAB/RABB-MMT (7 phr),(f) CIIR/CTAB/RABB-MMT (10 phr).

TEM was an effective and direct tool for observing the morphology of nano-composites and exfoliating nanostructures [44].Fig.11(e) and (f) showed the TEM image of CIIR/OMMT nanocomposites containing 5 phr and 7 phr OMMT,respectively.As shown in Fig.11(e),the OMMT(black lines)were evenly inserted and dispersed in the CIIR matrix.The red circle in Fig.11(e) was a peeled single-layer sheet OMMT,which can be seen in the CIIR matrix the dispersion was more extensive and uniform.The peeled layered structure was inserted into the CIIR chain,causing the orientation of the CIIR chain to change,which was mostly manifested as entanglement and surrounding the OMMT layer [45].This allowed the polymer chains between the OMMT layers to interact with the polar groups of the CIIR chains,which can dissipate more energy under the action of external forces.When the OMMT content reached 7 phr,a multi-layered structure of OMMT that was more aggregated occurred in the CIIR matrix shown in Fig.11(f).The results observed by TEM showed the same trend as the results observed by SEM and POM.When the content of OMMT in the CIIR matrix was high,the layers of OMMT may also aggregate under the effect of polarity.In the absence of external force,they can spontaneously gather together,making the OMMT in the CIIR matrix unevenly dispersed and macroscopically larger particles or OMMT multilayer structures appeared[14].This was also the main reason for the decrease in the damping performance of CIIR/OMMT nanocomposites [34].

Fig.12 showed the cross-section of different composite materials observed under POM.In Fig.12(a),when no filler was added,the cross-section of the composite material was very smooth,and fatigue cracks with concentrated stress were clearly visible.In Fig.12(b)and(c),as the amount of CTAB/RABB-MMT increased,the composite gradually exhibited broken marks and cracks.When the amount of this filler was increased to 5 and 7 phr (Fig.12(d)and (e)),the fracture surface showed a rough cliff structure,thus reducing the stress concentration to a certain extent.Under the action of external force,the pores may be extruded and deformed,and the mechanical energy was converted into thermal energy.As a result,high damping performance can be achieved in these composites.In Fig.12(f),gradually enlarged particles like black dots exhibited [33].This was due to the aggregation of CTAB/RABB-MMT to a certain extent,which was also reflected the tensile strength and elongation at break of the composite material [12].

Fig.13.Damping mechanism of CTAB/RABB-MMT in different CIIR composites.

3.6.Reinforcing and damping mechanisms of CIIR/OMMT composites

Fig.13 showed the effect of CTAB/RABB-MMT on the damping mechanism in CIIR composites.During the modification process of MMT,the surface condition and spatial level of the material had significantly changed.After the modification process of MMT through the two-step method,the interlayer spacing of MMT became larger,and a large amount of long-chain quaternary ammonium salt was grafted on the surface.On the one hand,this dramatically increased the dispersion of MMT in the rubber matrix[46].On the other hand,the interaction between nanoclay and rubber changed the properties of the material.This may be resulted from the addition of CTAB/RABB-MMT nanoparticles with high aspect ratio and surface area,which played an essential role in the adsorption and chemical bonding of hydroxyl groups on the surface of the CIIR matrix.At the same time,there was a stronger interaction between CIIR and MMT modified by CTAB/RABB,and there were some hydrogen bonds in the polymer matrix.As the filler content was increased (7 and 10 phr),more hydrogen bonds may appear in the polymer matrix.In this structure,due to the attraction of hydrogen bonds and van der Waals forces,many CIIR molecular chains were entangled and adsorbed on the surface of MMT.Thus,damping factor tanδ increased by about 37.9%.The highly exfoliated lamella structure of OMMT was dispersed in CIIR rubber,and its nano-scale dispersion in the polymer matrix led to the huge interface contact between the polymer and the inorganic filler.This may realize the combined action of the hydrogen bonds on the polymer and OMMT.Therefore,when the rubber was subjected to external forces,it can consume more mechanical energy and convert it into heat energy.This can improve the damping effect of the composite materials[35,47].In summary,after adding OMMT,the overall damping performance of the composite materials can be enhanced to a certain extent.

4.Conclusions

In this work,a simple method was proposed to prepare CIIR/OMMT composite materials,which were processed by using twostep modification of MMT.A novel type of OMMT,which showed obvious effect on improving the dynamic mechanical properties of CIIR composites,was successfully prepared.XRD results illustrated that the layer spacing of OMMT was increased significantly.TEM and SEM observation showed the lamella structure and the exfoliated nano-monolayer structure of OMMT,confirming the effectiveness of the two-step ultrasonic pulverization for exfoliating layered silicates.These nanoparticles owned the best dispersion performance in the CIIR matrix when the loading was 5 phr.DMA analysis revealed that as the OMMT load increased,the damping performance of the composite material gradually improved and reached the highest damping performance with a loading of 5 phr.At the same time,the tensile strength also exhibited a regular increase,and the elongation at break of the composite material decreased regularly as expected.The damping factor tanδmaxof the composite material was 37.9% higher than that of pure CIIR,and the position of the tanδ peak shifted toward high temperature,that is,from-22°C of pure CIIR to-7°C of the composite material.The mechanism for its better damping performance was owed to the hydrogen bonding between the substrates.These bonds may cause many CIIR molecular chains to be entangled and adsorbed on the surface of organoclay,which may cause further dissipation of mechanical energy and significant improvement in its damping performance.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51873103),Capacity Building Project of Some Local Colleges and Universities in Shanghai(17030501200),Scientific and Technological Support Projects in the Field of Biomedicine(19441901700),Talent Program of Shanghai University of Engineering Science (2017RC422017),and First-rate Discipline Construction of Applied Chemistry (2018xk-B-06).